1. Field of the Invention
This invention relates to transmission systems for transmitting signals, such as remote control signals and/or data signals, over electrical power distribution networks, and to transmitters and receivers for use in such transmission systems.
2. Description of the Prior Art
It has been proposed that such functions as tariff-switching, load-shedding and remote meter reading in electrical power distribution networks could be effected by means of radio-frequency carrier signals, which are modulated with digital signals and then transmitted over the wires of the network by modulating the network supply voltage with the modulated carrier signal. Our co-pending U.S. patent application Ser. No. 234,533, filed simultaneously herewith, describes and claims a receiver suitable for receiving such signals, even when they are relatively weak and heavily contaminated by noise.
However, the aforementioned proposal suffers from the serious drawback that for any given carrier frequency, and in a transmission system where respective transmitters located in or adjacent individual power consumer's meters are arranged to transmit data such as the meter reading to, for example, the power supplier's sub-station, the receiver may be located at a null in the network with respect to one or more of the transmitters: similarly, in a transmission system where a transmitter is arranged to transmit signals from, for example, the power supplier's sub-station to respective receivers located in or adjacent individual power consumer's meters, one or more of the receivers may be located at a null in the network with respect to the transmitter. The drawback is made worse by the fact that the positions of such nulls change continually as the transmission characteristics of the network vary in response to the continually changing load on the network.
It is an object of the present invention to provide transmission systems for transmitting signals over electrical power distribution networks, and transmitters and receivers for use in such systems, in which the effects of the abovementioned drawback are substantially alleviated.